Retarding systems and application to oil well cementing

ABSTRACT

Systems for retarding setting in an oilfield well cement slurry comprise a solution of a phosphonate selected from methylene phosphonic acid derivatives and a phosphate, in particular selected from the following salts or the corresponding acids: mono-phosphates (ortho-phosphates PO 4   2 ; meta-phosphates PO 3 ), acyclic poly-phosphates (pyrophosphates P 2 O 7   4 , tripolyphosphates P 3 O 10   5 ) or cyclic poly-phosphates. The system preferably may also comprise a retarder booster and is more particularly applicable to cementing at low or medium temperatures.

The present invention relates to drilling techniques for oil, gas,water, geothermal or analogous wells. More precisely, the inventionrelates to an additive, and to compositions including the additive, forcement slurries, more particularly for cementing a casing in an oil wellor the like.

After drilling an oil well or the like, a casing or a coiled tubing islowered into the well and cemented over all or a portion of its length.Cementing can in particular prevent fluids being exchanged between thedifferent formation layers through which the well passes, it can preventgas from rising via the annular space surrounding the casing, or it canlimit the ingress of water into the production well. Of course, it alsohas the principal aim of consolidating the well and protecting thecasing.

While a cement slurry is being prepared, then injected into the well,and finally positioned in the zone to be cemented, it must haverelatively low viscosity and practically constant rheologicalproperties. In contrast, once it is in position, an ideal cement wouldrapidly develop high compressive strength so as to enable other work inthe well under construction to be resumed, in particular to enabledrilling to be continued.

In practice, practically all cementing slurries are formulated with anadditive which retards setting of the cement, normally known as aretarder. The most widely used retarders are lignosulfates,hydroxycarboxylic acids such as citric acid, glucoheptonic acid orgluconic acid, saccharides or polysaccharides such ascarboxymethylhydroxyethyl cellulose, and organophosphates.

In practice, selecting a retarder depends on the temperature at the wellbottom, the cement slurry circulation temperature, and the presence orabsence of other additives with which the retarders may be incompatible.The majority of known retarders are effective only in a relativelynarrow temperature range, a fact which is more critical as thetemperatures to which the cement slurries are subjected are not alwaysprecisely known. A further difficulty is high sensitivity to variationsin concentrations of retarder or of other additives, and occasionallyalso to the cements used.

Under such conditions, formulating a cement slurry which is suitable forevery eventuality remains a particularly difficult art, all the more sosince oil well cements are, by their very definition, used on siteswhich are usually far from the facilities of an industrial laboratoryand which generally do not have access to the complete range ofavailable additives.

The present invention aims to provide a novel retarding system which issuitable for low/medium temperature applications, namely typically 70°C. to 140° C., and which is compatible with additives which arecurrently used for oilfield cements such as latexes, chemicallycross-linked polyvinyl alcohol type fluid loss control agents (inparticular those described in U.S. Pat. No. 5,594,050) and for which theeffect on a cement slurry is readily predictable, and in particularwhich has low sensitivity to variations in cement quality.

This aim is satisfied in the present invention by a system constitutedby a solution of a phosphonate selected from derivatives of methylenephosphonic acid, and a phosphate.

The phosphates may be mono-phosphates (ortho-phosphates PO₄,meta-phosphates PO₃) or acyclic poly-phosphates (pyrophosphates P₂O₇ ⁴,tripolyphosphates P₃O₁₀ ⁵), or cyclic poly-phosphates. Salts can also beused, for example, preferably sodium or potassium salts, or the acids(if they exist) of the following compounds: orthophosphoric acid H₃PO₄,sodium dihydrogen phosphate NaH₂PO₄, sodium monohydrogen phosphateNa₂HPO₄, trisodium phosphate Na₃PO₄, pyrophosphoric acid H₄P₂O₇, sodiumtripolyphosphate Na₅P₃O₁₀, and sodium cyclotrimetaphosphate Na₃P₃O₉.

The preferred retarding system of the invention is obtained with acalcium and sodium salt of ethylenediamine-N,N,N′,N′-tetrakis(methylene)phosphonic acid or the pentasodium salt of ethylenediaminetetra(methylenephosphonic) acid, associated with an orthophosphate.

The phosphonate to phosphate weight ratio is preferably in the range 2to 4, more preferably in the range 3 to 3.5.

The retarding system of the invention is suitable for applicationsbetween 50° C. and about 140° C. In a variation of the invention, theretarding system also comprises a retarder booster which can extend therange of application of the retarder of the invention to mediumtemperatures. For reasons of increased compatibility with otheradditives, and in particular fluid loss control additives, it ispreferable to use as a retarder booster a mixture of lignosulfates andhydroxycarboxylic acids (such as gluconates), but other conventionalhydroxycarboxylic acid-based retarder boosters or lignosulphates canalso be used. It should be noted that these retarder boosters arethemselves retarders but of quite low efficiency and thus are usuallyused in combination with other retarders. In this optimised variation ofthe invention, the retarding system is constituted by 40% to 45% oforthophosphoric acid, 10% to 15% of the calcium and sodium salt ofethylenediamine-N,N,N′,N′-tetrakis(methylene)phosphonic acid and 40% to50% of retarder booster, the percentages being by weight.

The retarding system of the invention can also contain a biopolymerwhich can improve the rheology of the cement slurry by minimizingsettling problems for systems with a high concentration of the retardingsystem.

In contrast to the numerous conventional retarders, the retarding systemof the invention is advantageously compatible with different types offluid loss control agents or gas migration agents, in particular withlatexes, and can be used in slurries in which seawater is used as themixing water.

The following examples illustrate the invention without limiting itsscope.

Except where otherwise indicated, the tests were carried out using asystem comprising a calcium/sodium salt of ethylenediaminetetramethylene phosphonic acid comprising (2.5 calcium per 3 sodium permole) which is commercially available from MONSANTO under the trade nameDEQUEST 2047 (the “phosphonate”) and an orthophosphoric acid fromPROLABO (the phosphate or H₃PO₄) with a purity of 99.999%, in 85%solution in water. The cement used was a class G oilfield cement(Dickerhoff North G).

EXAMPLE 1

60 ml of a cement slurry was prepared with a density of 1.893 g/cm³, andwith a water/cement volume ratio of 0.44. The phosphonate and phosphatewere added to the mixing water before the cement. After stirring for 35seconds at 4000 revolutions per minute, 3.5 g of slurry was weighed outand introduced into a calorimeter at 85° C. The time betweenintroduction into the calorimeter and the maximum hydration peak wasmeasured as a function of the percentage (calculated from the weight ofcement) of phosphonate and orthophosphoric acid and is shown in Table 1.

TABLE I N° % phosphonate % H₃PO₄ Time 1 0 0 3:20 2 0.05 0 5:00 3 0.075 03:45 4 0.05 0.1 11:00  5 0.05 0.15 4:10 6 0.05 0.05 5:20 7 0 0.15 3:45 80 1.2 1:15

While the calorimetric tests only indirectly reflect cement setting, itcan be seen that in the absence of phosphonate, the orthophosphate actsas an accelerator or a very slight retarder depending on theconcentration and temperature.

The phosphonate alone acts as a retarder up to a certain threshold butthere is clearly a synergistic effect when the phosphonate is used incombination with the orthophosphate. This table also shows the need foroptimisation of the phosphonate/phosphate ratio.

EXAMPLE 2

The same protocol was used, this time using mixtures of phosphonate andpyrophosphoric acid for setting at 85° C.

TABLE II N° % phosphonate % H₄P₂O₇ Time (hh:min) Intensity (mW)  9 0  0     3:30 175  10 0.1 0    11:40 105  11 0.1 0.0004 16:00 80 12 0.10.0006 11:30 90 13 0.1 0.0012 13:10 85

In similar fashion to orthophosphoric acid, Table II above shows thatthere is a critical ratio at which a synergistic effect exists betweenthe retarding effect of the phosphonate and the retarding effect of thephosphate.

EXAMPLE 3

The tests were repeated at 111° C. (231° F.) and by adding 35% (byweight of cement) of silica flour as is usual from such temperatures toprevent retrogression of the compressive strength and an increase in thepermeability of the set cement. The water to cement ratio was kept at0.44, the slurry density was 1.797 g/cm³. The same protocol as thatdescribed above was used except that the silica flour was mixed withwater for 15 seconds before cement addition was commenced.

In Table III below, a*after the time indicates the presence of twohydration peaks, the maximum peak reported here corresponding to thesecond hydration peak. A stark reduction in peak intensity (of the orderof 50% or more) could be associated with setting spread over a longperiod, which is not desirable for a good retarding system.

TABLE III Test % phosphonate % H₃PO₄ Time (hh:min) Intensity (mW) 14 0 02:05  125 15 0.05 0 3:20  105 16 0.075 0 4:10  100 17 0.2 0 6:40*  70 180.4 0 10:00   2 19 0 0.1 2:30* 105 20 0 1.2 1:10*  95 21 0.05 0.05 2:30*110 22 0.05 0.15 2:30*  90 23 0.05 1.2 1:25* 100 24 0.2 0.008 5:30*  90

It can be seen that the orthophosphate retained a very strongaccelerating effect, even at relatively low concentrations. As thephosphonate concentration increased, the intensity of the principalhydration peak strongly diminished until it practically disappeared,i.e., the cement no longer set but behaved as a gel.

These calorimeter cell tests have thus shown that a synergistic effectexists for the retarding effect when the phosphonate/phosphate ratioequals a certain value. They also show that there is a criticalthreshold for the concentration of phosphonate in the cement slurry,above which threshold cement setting is affected.

EXAMPLE 4

Following the calorimetric tests, the authors of the present inventionsought to optimize a three-component retarding system: a phosphonate(the calcium/sodium salt of ethylenediamine tetramethylene phosphonicacid used in the preceding examples), orthophosphoric acid and aretarder booster comprising 11.5% (by weight) of sodium gluconate, 76%of modified sodium lignosulfonate and 6.5% of tartaric acid.

These tests were carried out in 3 temperature ranges: between 68° C. and91° C. (low temperature range); between 91° C. and 113° C. (mediumtemperature range) and between 113° C. and 138° C. (high temperaturerange).

It should be noted that these temperatures correspond to temperatures ata well bottom of a slurry pumped from the surface. It should also benoted that this temperature is normally lower than the temperature atwhich cement setting occurs such that in practice, the compressivestrengths of the set cement in the well are higher than the valuesmeasured during these tests. These temperature ranges are encountered inlow or medium temperature wells.

The experimental data are shown in Tables IV, V and VI below in whichthe concentration of retarder (active matter) is given with respect tothe weight of cement. The proportions by weight of orthophosphoric acidand phosphonate were varied between 10% and 62% and 1% and 31%respectively (the percentage by weight of retarder booster, being thecomplement to 100% not being shown in these tables). For all of the testslurries, the quantity of water was adjusted to obtain a fixed densityof 1.89 g/cm³.

For the low temperature tests, 0.03% (with respect to the weight ofcement) of a biopolymer. and 0.1 gallons per sack of cement of a normaldispersing agent, in this case an aqueous solution of polynaphthalanesulfonate (PNS), were added, (i.e., 0.1 U.S. gallons (3.78 litres) per42 kilogram sack, 0.1 gps=9 cm³/Kg of cement).

The medium and high temperature tests were carried out without addingpolynaphthalene sulfonate but with 0.065% of biopolymer (such as“BIOZAN” produced by the Kelco Oilfield Group), acting as ananti-settling agent. Further, compositions comprising 35% (with respectto the weight of cement) of silica flour were used for these tests.

The slurries were prepared and the measurements carried out usingprotocols recommended by the API (American Petroleum Institute), usingthe following order for adding the additives: preparing the mixing waterby successive addition of phosphonate, orthophosphoric acid, ifnecessary an antifoam agent and the dispersing agent, then adding apremixed dry solid material mixture (cement, biopolymer and silicaflour).

The thickening time corresponded to the production of a consistency of100 BC, measured in standardized BC units. The transition timecorresponded to passing from a consistency of 30 BC to a consistency of100 BC.

The time required to attain a compressive strength of 50 psi (poundsforce per square inch) (i.e., 345 kPascals) and 500 psi (3450 kpascals)was also measured for each slurry. Further, the compressive strengthafter 24 hours was measured (1 psi=6.894 kPascals). A value of the orderof 2000 psi (13.8 Mpascals) after 24 hours is generally judged to besatisfactory for the envisaged applications.

A further series of measurements was made on the slurry rheology, at themixing temperature and at the circulation temperature T in the well fortemperatures below 85° C. or at 85° C. for higher temperatures. The datameasured in this instance were the plastic viscosity (in milliPascalssecond or centiPoises) and the yield point (expressed, as is customaryin this art, in lbf/100ft²=0.4787 Pascals). Compositions with lowviscosity and low yield point are desirable.

Measurements were also carried out on the gel strength developed whenthe composition was left to rest for 10 minutes (in lbf/100ft²). Thesemeasurements were carried out at the temperature of slurry circulationat the well bottom. Because the formulations in Tables VI and VIIcontained no dispersing agent, the relatively high gel values obtainedafter 10 minutes of rest are not significant. Further, the gel waseasily broken up as shown by the results after one minute's stirring at3 revolutions per minute (gel+i minute).

Finally, the last column shows the volume of free water (indicatingsettling in certain phases of the slurry) formed for 250 ml of slurry(values approaching zero were desirable).

These tests enabled a preferred ternary retarding system to be selectedwhich comprised between 39% and 45% of orthophosphoric acid, 10% to 15%of phosphonate and 40% to 51% of retarder booster, (modified sodiumlignosulfonate/sodium gluconate). With this optimum composition, theretarding effect on cement setting was good, with a relatively monotonicresponse depending on the concentration of the retarder added, andrelatively low sensitivity to temperature variations; low sensitivity toshear (characterized by a short time between the time to obtain 100 BCand that to obtain 50 psi) and rapid development of compressive strength(rapid passage from 50 to 500 psi) and good compressive strength after24 hours.

For the medium temperature range, an optimum was discerned forcompositions comprising 40.5% of orthophosphoric acid, 12.25% ofphosphonate and 47.25% of retarder booster.

For improved slurry stability, in particular at the highesttemperatures, the preferred retarding system of the invention alsocontains at least 2%, preferably 3% (with respect to the weight of dryretarder) of biopolyrners.

The retarder was prepared in the form of an aqueous solution, preferablyrelatively concentrated, a concentration by weight in the range 15% to25% being a good compromise to obtain both a relatively low retarderviscosity (in particular to facilitate on-site mixing) and thepossibility of using only relatively low concentrations. In thefollowing, the concentration of retarder in the aqueous solution wasfixed at 16.9%, corresponding to a density of 1.07 g/ml.

EXAMPLE 5

Table VII below shows the performance of the retarder selected followingthe experiments described in Example 4, namely a solution containing16.9% of a mixture constituted by 40.5% of orthophosphoric acid, 12.25%of phosphonate and 47.25% (percentage by weight) of a retarder boosterbased on sodium gluconate and modified sodium lignosulfonate and adding3% of biopolymers (with respect to the weight of retarder activematter).

For these tests, an antifoam agent was systematically used and, for lowtemperatures, a polynaphthalene sulfonate type dispersing agent. Fortests carried out at over 100° C., the slurries were formulated with 35%of silica (percentage per weight of cement BWOC).

TABLE VII Temperature T (° C.) 68.3 76.7 85 90.6 101.7 110 115.6 126.7137.8 Density (g/cm³) 1.893 1.893 1.893 1.893 1.893 1.893 1.893 1.8931.893 Retarder (cm³/kg) 9.9 10.8 22.5 40.5 68.7 81.4 87.3 119.7 137.7Dispersing agent (cm³/kg) 5.85 4.5 5.4 3.6 — — — — — Antifoam agent(cm³/kg) 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 silica (% BWOC) — — — — 3535 35 35 35 Rheology at 20 ° C. Viscosity 24 26 28 34 63 77 87 109 116(mPa · s) Ty (lbf/100 ft²) 0.5 1.8 2.5 1.6 8.3 10 11 16 26 Rheology at TViscosity 14 14 16 35 36 40 45 62 59 (mPa · s) Ty (lbf/100 ft²) 3.6 9.55.9 3.7 5.9 4.5 7.3 11 10 gel After 10 min. 25 29 45 20 10 8 12 13 10 +1min at 3 rpm 7 10 5 4 5.5 3 6 5.5 4 Free water (ml) 0.7 2 0.1 0 0.2 0.050.05 0.05 0.05 Time to 100 BC (hrs:min)  6:09  3:53  5:29 5:47 6:05 5:005:17  7:41  8:43 Transition 30 → 100 BC 25 17 14 35 29 5 5 5 5 (min) 50psi (hrs:min) 11:18  9:19  8:41 7:55 8:37 7:36 7:25 12:11 14:13 500 psi(hrs:min) 12:53 10:22 10:02 9:16 9:52 9:01 8:59 14:47 16:46 CS 24 hrs(psi) 2350 2587 2783 3287 2334 1948 1999 1369 1665

It can be seen that the performance obtained was highly satisfactoryover the entire range of test temperatures. In particular, a very shorttransition time was noted between a consistency of 30 BC and 100 BC witha time period between the time to obtain 50 psi (345 kPascals) and 500psi (3450 kPascals) generally of the order of 60-90 minutes.

EXAMPLE 6

On site, it is very difficult to adhere precisely to the prescribedadditive concentrations. The usual retarders are sometimes extremelysensitive to small differences in concentration as is shown in TableVIII below, where a variation in retarder concentration of the order ofonly 0.2 liters per sack of cement can cause a variation in thethickening time (arbitrarily defined as the time to obtain a consistencyof 100 BC) of more than 4 hours, or even more than 16 hours with certainretarders.

Further, certain conventional retarders are highly sensitive totemperature variations. As the tests carried out with a 38% refinedlignosulfonate solution show, a temperature difference of the order of10° C. can cause a variation in thickening time by a factor of 2.

By comparison, the retarder of the invention has a lower sensitivity tovaciations in concentrations and temperatures.

TABLE VIII Concen- Time to tration 100 BC T (° C.) Retarder (cm³/kg)(hrs:min) 85 50% solution of partially modified 3.5 4:10 calcium andsodium lignosulfonate 7 22:46  85 38% refined lignosulfonate solution3.5 2:26 7 8:23 96.1 38% refined lignosulfonate solution 7.7 3:34 96.118% calcium glucoheptonate solution 2.8 4:00 6.3 >10:00    85 Retarderof invention 10.6 4:20 14.1 6:00 96.1 Retarder of invention 40.8 3:3044.3 6:00 101.5 Retarder of invention 50 3:30 53.5 6:00 107.2 Retarderof invention 58.4 4:30 61.9 6:00

EXAMPLE 7

A further constraint is due to supplies from cement works andcompositional variations which are encountered between cements fromdifferent sources.

Some conventional retarders are highly sensitive to these differenceswhich is absolutely not the case with the retarding system of theinvention, as shown in Table IX. For these tests, the temperature wasfixed at 101.7° C., the slurry density at 1.89 g/cm³ and theconcentration of retarder at 53.7 cm³ per kilo of cement. The slurriesalso contained 2.1 cm³ of antifoam agent per kilogram of cement and 35%(with respect to the weight of cement) of silica flour.

TABLE IX Dyckerhoff Cement type: North LoneStar Saudi Rheology at 20° C.Viscosity (mPa · s) 75 71 76 Ty (lbf/100 ft²) 10   8.5 15 Rheology at85° C. Viscosity (mPa · s) 37 33 30 Ty (lbf/100 ft²)   6.2   6.5   8.5gel After 10 min.   11.5 15 29 + 1 min (at 3 rpm).   5.5  8 11 Freewater (ml) traces traces traces Time to 100 BC (hrs:min) 6:05 6:20 5:14Transition 30 → 100 BC (min) 29  8 33 50 psi (hrs:min) 8:37 8:41 6:59500 psi (hrs:min) 9:52 9:59 9:52 C.S. 24 hrs (psi) 2334  2080  2334 

EXAMPLE 8

This example ascertains the possibility of using a hydroxycarboxylicacid as a retarder. The preceding tests enabled the orthophosphoricacid: phosphonate: retarder booster ratio to be optimized at 60:18:22,the quantity of biopolymers remaining fixed at 3% of the quantity of drymatter in the retarder solution. Slurries were used with a density of1.89 g/cm³, adding 2.1 cm³ of antifoam agent per kilogram of cement. Forthe low temperature test, a 40% polynaphthalene sufonate solution wasused as the dispersing agent.

Table X, in which the concentrations of retarder are concentrations byweight with respect to the weight of cement, shows that thehydroxycarboxylic acid is perfectly suitable as a retarder booster andsometimes even produced superior results, however gelling problems wereencountered with certain fluid loss control agents.

TABLE X Temperature: 68.3° C. 101.7° C. 137.8° C. Retarder (%) 0.07 0.71 Dispersing agent 5.6 — — (polynaphtalene sufonate) (cm³/kg) Silicaflour (% BWOC) — 35 35 Rheology at 20° C. Viscosity (mPa · s) 22 62 70Ty (lbf/100 ft²) 1.6 9 12 Rheology at T° C. Viscosity (mPa · s) 14 34 31Ty (lbf/100 ft²) 6.1 10 14 gel After 10 min. 15 20 18 + 1 min (at 3rpm). 4 11 6 Free water (ml) 2 0.6 0.35 Time to 100 BC (hrs:min)  5:386:01 5:23 Transition 30 → 100 BC (min) 17 30 9 50 psi (hrs:min) 10:288:48 8:56 500 psi (hrs:min) 11:45 10:02  10:37  C.S. 24 hrs (psi) 24712472 3152

EXAMPLE 9

This example ascertains that a mixture of a hydroxycarboxylic acid and alignosulfonate can be used as a retarding agent. All of the tests werecarried out at 137.8° C., with a slurry with a density of 1.89 g/cm³,adding 2.1 cm³ of antifoam agent per kilogram of cement and 35% ofsilica flour (BWOC). The performance obtained (Table XI) was again verysatisfactory.

TABLE XI H₃PO₄/ Phosphonate/ retarder booster: 60/18/22 60/18/2240.5/12.5/47.2 40.5/12.5/47.2 Retarder (%)  1  2   0.7  1 Rheology at20° C. Viscosity 83 123  72 84 (mPa · s) Ty (lbf/100 ft²) 19 33 14 12Rheology at 85° C. Viscosity 43 58 36 40 (mPa · s) Ty (lbf/100 ft²) 1925 12 13 gel After 10 min. 49 23 17 15 + 1 min 12 19 11  9 (at 3 rpm)Free water (ml)   2.5  0   1.2   0.5 Time to 1:03 3:19 3:24  9:17 100 BC(hrs:min) Transition  3  4  5  6 30 → 100 BC (min) 50 psi not 7:43 not11:20 (hrs:min) measured measured 500 psi 8:55 13:20 (hrs:min) C.S. 24hrs 4115  2972  (psi)

EXAMPLE 10

Since the optimized retarding system of Example 5 was particularlyoptimized for low temperatures, it was ascertained that it could also beused with a slurry containing plaster, more particularly used fordeepwater drilling at very low temperature. The slurries, tested at 20°C., had a density of 1.89 g/cm³, contained 5.3 cm³ of antifoam agent perkilolgram of cement, 7 cm³ (per kilo of cement) of dispersing agent(polynaphthalene sufonate in 40% solution) and 150% of plaster (withrespect to the weight of cement).

TABLE XII Retarder concentration (cm³ per kilo of cement): 28.1 42.228.1 Rheology after mixing at 20° C. Viscosity (mPa · s) 99 119 99 Yieldpoint Ty (lbf/100 ft²) 6.0 11.0 6.0 Rheology after completion at 20° C.Viscosity (mPa · s) 113 134 149 Yield point Ty (lbf/100 ft²) 6.0 8.0 7.1gel After 10 min. 13 19 16 + 1 min (at 3 rpm) 9 10 9 Free water (ml) 0 00 Time to 100 BC (hrs:min) 2:05 4:30 2:18 Transition 30 → 100 BC (min))30 56 15 Compressive strength (psi) After 5 h — 830 889 After 8 h — 735— After 2 days — 1079 822 After 3 days — 1517 983 After 7 days — 19821783

The retarding system of the invention enabled a compressive strength ofmore than 500 psi to be obtained after only 5 hours, with a thickeningtime of 4:30 at 20° C. and 2:18 at only 10° C. This shows gooddevelopment of compressive strength.

EXAMPLE 11

This retarding system was also tested at 20° C. for systems basedexclusively on plaster (Table XIII). The system density was 1.8 g/cm³,comprised 2.1 cm³ of antifoam agent per kilogram of plaster and 3.5 cm³of retarder per kilo of plaster.

TABLE XIII Rheology after mixing at 20° C. Viscosity (mPa · s) 84 YieldpointTy (lbf/100 ft²) 22 Rheology after completion at 20° C. Viscosity(mPa · s) 61 Yield pointTy (lbf/100 ft²) 14 gel After 10 min. 14 + 1 min(at 3 rpm).  8 Free water (ml)  0 Time to 100 BC (hrs:min) 3:40Transition 30 → 100 BC (min))  5

It can be seen that the retarding system of the invention is alsocapable of retarding a system based exclusively on plaster.

EXAMPLE 12

The Dequest 2047 system was replaced by the pentasodium salt ofethylenediamine tetra(methylenephosphonic) acid, commercially availablefrom Monsanto under the trade name Dequest 2046. The retarder wasconstituted by a 17.5% solution of a mixture of 38.8% of orthophosphate,15.97% of phosphonate and 45.25% of retarder booster (percentages byweight) and 2.87% of biopolymers (with respect to the weight of retarderactive matter). The slurry density was 1.893 g/cm³, comprising 35% (byweight of cement) of silica flour and containing 3 cm³ of antifoam agentper kilogram of cement, the concentration of retarding agent being 68.4cm³ per kilogram of cement. The measurements were carried out at 101.7°C.

The performances indicated in Table XIV are remarkable; the very longthickening time should in particular be noted, but the transition timewas only 36 minutes and the compressive strength developed very rapidly.The absence of calcium thus appears to have a favorable effect.

TABLE XIV Rheology after mixing at 20° C. Viscosity (mPa · s) 89 YieldpointTy (lbf/100 ft²) 9 Rheology at 85° C. Viscosity (mPa · s) 47 YieldpointTy (lbf/100 ft²) 6 Gel After 10 min/+1 min at 3 rpm. 7/2.5 Freewater (ml) 0.05 Time to 100 BC (hrs:min) 14:26 Transition 30 → 100 BC(min)) 36 Time to 50 psi (hrs:min) 14:33 Time to 500 psi (hrs:min) 15:54CS after 24 hours 21.07 MPa (3057 psi)

EXAMPLE 13

Based on example 12, the retarding system constituted by the pentasodiumsalt of ethylenediamine N,N,N′,N4 tetramethylene phosphonic acid (EDTMP)and phosphoric acid has been further studied.

Cement slurries have been prepared following the API procedure using acement from Dyckerhoff (North, Black Label), a dispersing agent (0.03gal/sk, 2.66 ml/kg of cement), an anti-foam agent (0.03 gal/sk, 2.66ml/kg of cement), 35% of silica flour (BWOC) and the retarding systemEDTMP acid/phosphoric acid (0.2gal/sk, 17,77 ml/kg of cement for thetests reported table XV and 0.4 gal/sk, 35,51ml/kg of cement for thetests reported table XVI below).

A water solution of pentasodium EDTMPS salt is used and has an activematerial contenftbf 25 wt. %. The phosphoric acid is a 85% solution. Thedifferent ratios for EDTMP acid/phosphoric acid that have been testedare based on a 30 wt. % retarder concentration in water solution. Theratios have been calculated using the mathematical system below:$\begin{Bmatrix}{\frac{0.25x}{0.85y} = z} \\{{x + y} = 30}\end{Bmatrix}$

where x is the concentration of EDTMP acid in wt. %, y is theconcentration of the phosphoric acid in wt. % and z the desired ratioEDTMP acid/phosphoric acid.

TABLE XV z 0.4 0.50 0.59 0.70 0.8 0.9 Mixing rheology Pv (mPa · s) 47 4238 40 39 43 Ty lbf/100 sqft 13 6 5 6 6 6 10″gel (lbf/100 sqft) 10 5 5 66 6 API rheology @ 85° C. 32 37 32 28 41 39 PV(mPa · s) Ty 18 26 37 3223 25 10″ gel 9 11 12 14 11 12 10′gel/1′stirring 18/13 12/9 14/13 12/710/7 11/8 API Free water (ml) 6.7 3.0 4.3 3.6 3.2 2.6 Thickening Time @85° C. Time to 100 BC 1:56 8:52 8:44 11:33 15:38 >15.5 h (hrs:min)Transition 30 → 100 0:22 0:35 0:53  0:58  1:06 — (hrs:min) ThickeningTime @ 104.4° C. Time to 100 BC 1:00 1:37 2:30  3:40  3:54 5:51(hrs:min) Transition 30 → 100 0:15 0:16 0:23  0:25 0:30 0:45 (hrs:min)

TABLE XVI z 0.50 0.59 0.70 0.8 0.9 Mixing rheology Pv (mPa · s) 41 42 3943 42 Ty lbf/100 sqft  3  4 4 4  6 10″gel (lbf/100 sqft)  4  4 4 5  6API rheology @ 85° C. PV(mPa · s) 23 27 27 24 26 Ty 13 18 19 19 19 10″gel  7  8 8 10 10 10′gel/1′stirring 13/9 15/13 16/11 10/8 12/10 API Freewater (ml) 18  8 7.5 5.3  6 Thickening Time @ 104.4° C. Time to 100 BC(hrs:min) 7:11 9:30 10:43 12:04 13:22 Transition 30 → 100 0:20 0:23 0:27  0:22  2:57 (hrs:min)

From table XV and XVI, trends can be observed. First, as the ratio ofEDTMP acid increases from 0.4 to 0.9, the thickening time increases.Second, the free water decreases with increasing EDTMP acid ratio.Third, the absolute transition time from 30 bc to 100 bc increases asthe EDTMP acid ratio increases. However, if the transition time iscalculated as a percentage of the thickening time a decrease is observedwith increasing EDTMP acid ratio in the range from 0.4 to 0.8.

At an EDTMP acid ratio of 0.9 at 104.4° C. (220° F.) and 36 cm³/kg (0.4gal/sk), the cement set is poor with a low strength. The set cement atthis condition could be broken with the force of the hands. An importantobservation is that all other samples have shown a “true” and hard setthat was difficult to clean out from the consistometer test cell.

Best results are obtained with a ratio ranging between 0.6 and 0.8,preferably of about 0.7, a good compromise between the thickening timeand the amount of produced free water, and a ratio which also assurethat the retarder is efficient event at high concentration without‘killing’ the cement.

TABLE IV Strength Rheology at Concen- Phos- Thickening Transition after24 Rheology circulation Gel Free Test tration T phonate H₃PO₄ time time50 psi 500 psi hours on mixing temperature at +1 water No % (° C.) % %hrs:min min. hrs:min hrs:min psi PV Ty PV Ty 10 min min ml 1 0.5 79.5 1636 20:54 45 23:10 27:19 0 57 2.2 34 4.1 21 6 0 2 0 79.5 16 36  3:41 31 8:33 10:06 1825 57 3.1 31 6.9 37 10 0 3 0.375 90.5 16 36  3:23 50  7:20 8:39 2483 59 2.6 38 6.3 52 15 0 4 0.125 68.3 16 36 10:56 30 20:28 23:23510 57 3.5 37 4.8 19 7 0 5 0.375 68.3 16 36 29:15 26 41:02 45:25 0 58 332 3.6 14 3 0 6 0.125 90.5 16 36  2:30 45  7:33  8:56 2497 48 1.5 26 1698 27 0 7 0.375 83 31 36  9:41 17 15:12 19:10 1348 36 3.3 33 6.4 31 13 08 0.125 75.8 1 36 13:03 26 22:19 24:44 0 62 3.6 37 5.3 26 8.5 0 9 0.37575.8 1 36 34:56 56 35:24 41:00 0 60 3.4 35 6.2 12 5.5 0 10 0.250 86.7 136 12:34 32  8:06  9:31 2931 62 2.6 38 18 112 36 0 11 0.125 83 31 36 9:32 78 10:25 12:13 2062 51 2.2 51 6.6 65 14 0 12 0.250 72 31 36 25:5060 30:58 34:49 0 54 5.6 37 5.9 20 8.5 0 13 0.375 83 19.75 62  3:27 24 8:24 10:12 2300 58 4.7 42 16 64 29 0 14 0.125 75.8 12.25 10 18:00 3433:34 36:41 0 56 2.3 34 6.2 26 9.5 0 15 0.375 75.8 12.25 10 64:40 60 >8days >8 days 0 62 2.4 42 5.8 34 11 0 16 0.250 86.7 12.25 10 35:43 6026:10 32:36 32 56 3 49 25 144 50 0 17 0.250 79.5 27.25 10 54:38 60 >8days >8 days 0 58 4 36 6.3 29 9 0 18 0.125 83 19.75 62  2:10 54  8:2610:06 2142 53 2.2 34 13 101 24 0 19 0.250 72 19.75 62  7:02 33  9:4912:15 1650 55 6.5 41 7.4 39 13 0 20 0.250 79.5 4.75 62  5:14 41 10:3312:33 2010 58 1.5 35 6.1 28 11 0 21 0.250 79.5 16 36 12:18 34 21:4725:38 480 47 5.3 39 8.7 27 12 0 22 0.250 79.5 16 36 12:26 24 16:10 18:361344 60 2.1 37 6.6 39 13 0 23 0.250 79.5 16 36 12:24 23 19:18 22:27 82449 3.6 33 5.7 27 10 0

TABLE V Strength Rheology at Concen- Phos- Thickening Transition after24 Rheology circulation Gel at Free Test tration T phonate H₃PO₄ timetime 50 psi 500 psi hours on mixing temperature At +1 water No % (° C.)% % hrs:min min. hrs:min hrs:min psi PV Ty PV Ty 10 min min ml 1 1.5101.7 16 36 16:53 60 17:01 19:26 1144 112 10 69 9 14 7.5 0.05 2 0.7101.7 16 36  2:54 28  7:22  8:21 2769 103 23 53 23 58 34 0.2 3 1.3 112.816 36  3:26 20  8:00  9:44 2676 96 15 61 11 11.5 6 0.05 4 0.9 42.2 16 3615:27 36 12:45 14:48 1927 100 18 57 21 6 9.5 0 5 1.3 42.2 16 36 35:43 2237:00 39:00 2 101 17 56 13 15 10 0.1 6 0.9 112.8 16 36  1:42 15  5:50 7:04 2941 100 19 69 33 27.5 7.5 0.1 7 1.3 105.6 31 36  9:04 18 14:0117:00 1750 82 14 51 14 19 10 0.05 8 0.9 97.8 1 36  8:39 40 13:47 15:132005 101 16 54 13 15 9 0.5 9 1.3 97.8 1 36 33:11 50 37:23 40:11 20 10915 60 10 8 5 0.05 10 1.1 108.9 1 36  5:30 21 11:12 13:05 2323 95 13 57 99 4 0.1 11 0.9 105.6 31 36  5:37 200  6:45  8:23 2249 95 20 45 30 85 170.05 12 1.1 94.4 31 36 26:06 100 33:56 39:00 0 90 12 60 22 44 22 0 131.3 105.6 19.75 54  2:19 21  5:29  6:32 2400 95 20 60 15 24 12 0 14 0.997.8 12.25 18 20:37 120 38:32 42:16 0 102 15 60 12 15 7 0 15 1.3 97.812.25 18 26:28 180 80:43 87:30 0 103 9 58 5 5 4.5 0.5 16 1.1 108.9 12.2518  4:10 4 18:34 21:28 0 100 9.5 55 6 9 5 0 17 1.1 101.7 27.25 18 30:1822 42:10 49:13 0 98 12 64 9 20 9 0 18 0.9 105.6 19.75 54  1:46 13  4:35 5:51 2700 100 25 57 21 31 15 0 19 1.1 94.4 19.75 54  4:12 25  8:2610:07 2200 101 23 57 24 60 14 0 20 1.1 101.7 4.75 54  3:10 24  5:45 6:54 2468 89 19 61 16 15 9 0 21 1.1 101.7 16 36  6:31 37  8:26  9:322652 93 12 54 10 18 8 0 22 1.1 101.7 16 36  5:58 33 10:12 11:33 2300 10316 56 14 17 14 0.2 23 1.1 101.7 16 36  5:27 47 10:05 11:36 2180 94 16 5513 13 10 0.2

TABLE VI Strength Rheology at Concen- Phos- Thickening Transition after24 Rheology circulation Gel Free Test tration T phonate H₃PO₄ time time50 psi 500 psi hours on mixing temperature at +1 water No % (° C.) % %hrs:min min. hrs:min hrs:min psi PV Ty PV Ty 10 min min ml 1 3 126.7 1636 18:38 24 24:38 27:47 50 134 18 69 73 75 2 0.05 2 1.5 126.7 16 36 3:07 4  5:31  6:50 2399 112 14 73 12 10 5 0.05 3 2.625 137.8 16 36 6:55 13  9:47 12:11 2150 129 11 72 6.6 6.5 2.5 0.1 4 1.875 115.6 16 36 9:58 15 10:11 11:50 2563 101 13 61 8 8 3 0.3 5 2.625 115.6 16 36 30:5245 23:11 27:32 50 129 11 72 6.6 6.5 2.5 0.1 6 1.875 137.8 16 36  2:15 18 5:56  7:32 2550 93 14 49 10 9.5 1 0.05 7 2.625 130.6 31 36  4:12 2110:21 15:04 2572 119 16 56 11 8.5 2 0.1 8 1.875 122.8 1 36  6:58 4 11:1113:18 2640 92 12 48 7.6 8 1 0.5 9 2.625 122.8 1 36 29:18 25 32:14 38:5I0 106 15 52 7.6 8 1 0.1 10 2.250 133.9 1 36  7:09 5  7:02  8:18 3484 10314 54 8.5 8.5 1.5 0.1 11 1.875 130.6 31 36  4:35 40  6:44  9:15 2393 10313 63 9.7 14 7 0 12 2.25 119.4 31 36  7:00 40 14:02 18:23 1042 125 18 7013 11 7 0 13 2.625 130.6 19.75 46  2:57 20  6:41  7:53 1100 111 22 67 108 5 0 14 1.875 122.8 12.25 26 11:42 6 13:47 16:37 2052 106 13 58 4.6 8 20.5 15 2.625 122.8 12.25 26 22:24 5 42:41 47:39 0 97 13 45 8 10 6 0.0516 2.250 133.9 12.25 26  7:18 21 11:12 13:27 1813 101 14 44 9.8 9.5 150.05 17 2.250 126.7 27.25 26 11:39 42 16:29 24: 8 490 93 11 54 6 5.5 20.05 18 1.875 130.6 19.75 46  2:05 17  5:50  7:08 2891 122 17 73 13 11.516 0.05 19 2.250 119.4 19.75 46  5:00 23  8:23  9:59 2733 117 24 73 1219 5 0 20 2.250 126.7 4.75 46 11:01 17 13:49 15:33 2213 135 19 61 1.7 62 0.6 21 2.250 126.7 16 36  6:37 18  9:59 12:16 2521 94 11 54 6 7.5 2 022 2.250 126.7 16 36  5:07 22 11:28 13:38 2161 103 12 61 5 6.5 2 0.05 232.250 126.7 16 36  7:53 20  4:55 17:21 1794 101 17 51 9 2 1 0.05

What is claimed is:
 1. A retarding system for a well cementing slurry,the system comprising a solution containing: a) a methyl phosphonic acidderivative phosphonate; and b) a phosphate.
 2. A retarding systemaccording to claim 1, wherein the phosphate selected from the groupconsisting of mono-phosphates, ortho-phosphates PO₄ ², meta-phosphatesPO₃, acyclic poly-phosphates, pyrophosphates P₂O₇ ⁴, tripolyphosphatesP₃O₁₀ ⁵, and cyclic poly-phosphates.
 3. A retarding system according toclaim 1, wherein the phosphate is selected from the group consisting oforthophosphoric acid H₃PO₄, sodium dihydrogen phosphate NaH₂PO₄, sodiummonohydrogen phosphate Na₂HPO₄, trisodium phosphate Na₃PO₄,pyrophosphoric acid H₄P₂O₇, sodium tripolyphosphate Na₅P₃O₁₀, and sodiumcyclotrimetaphosphate Na₃P₃O₉.
 4. A retarding system according to claim1, wherein the methylene phosphonic acid derivative phosphonate isselected from the group consisting of calcium and sodium salts ofethylenediamine-N,N,N′,N′-tetrakis(methylene) phosphonic acid, and thepentasodium salt of ethylenediamine tetra(methylenephosphonic) acid. 5.A retarding system according to claim 3, wherein the phosphate isorthophosphoric acid H₃PO₄.
 6. A retarding system according to claim 1,wherein the solution has a, phosphonate to phosphate ratio in the range2 to 4 by weight.
 7. A retarding system according to claim 1, furthercomprising a retarder booster selected from the group consisting oflignosulfonates, hydroxycarboxylic acids, and mixtures thereof.
 8. Aretarding system according to claim 1, further comprising a biopolymer.9. A retarding system according to claim 7, comprising: a) 40% to 45% byweight of orthophosphoric acid, b) 10% to 15% by weight of the calciumand sodium salt ofethylenediamine-N,N,N′,N′-tetrakis(methylene)phosphonic acid, and c) 40%to 50% by weight of retarder booster.
 10. A retarding system accordingto claim 1, comprising the pentasodium salt of ethylenediamine N,N,N′,N4tetramethylene phosphonic acid (EDTMP) and phosphoric acid.
 11. Aretarding system according to claim 10, wherein the weight ratioEDTMP/phosphoric acid is between 0.6 and 0.8.
 12. A retarding system asclaimed in claim 6, wherein the solution has a phosphonate to phosphateratio in the range 3 to 3.5 by weight.
 13. A retarding system as claimedin claim 11, wherein the weights ratio EDTMP/phosphoric acid is about0.7.
 14. A retarded well cement system for use at a temperature in therange 70° C. to 140° C., comprising: i) cement; ii) water; iii) aphosphonate derived from methyl phosphonic acid; and iv) a phosphate.15. A retarded well cement system as claimed in claim 14, wherein thecement is selected from the group consisting of Portland cement andplaster-based cement systems.
 16. A retarded well cement system asclaimed in claim 14, further comprising a retarder booster selected fromthe group consisting of lignosulfonates, hydroxycarboxylic acids, andmixtures thereof.
 17. A retarded well cement system as claimed in claim14, further comprising a biopolymer.
 18. A retarded well cement systemas claimed in claim 14, wherein the phosphonate to phosphate ratio liesin the range 2 to 4 by weight.
 19. A retarded well cement system asclaimed in claim 14, wherein the phosphate is selected from the groupconsisting of mono-phosphates, ortho-phosphates PO₄ ², meta-phosphatesPO₃, acyclic poly-phosphates, pyrophosphates P₂O₇ ⁴, tripolyphosphatesP₃O₁₀ ⁵, and cyclic poly-phosphates.
 20. A retarded well cement systemas claimed in claim 14, wherein the methylene phosphonic acid derivativephosphonate is selected from the group consisting of calcium and sodiumsalts of ethylenediamine-N,N,N′,N′-tetrakis(methylene) phosphonic acid,and the pentasodium salt of ethylenediamine tetra(methylenephosphonic)acid.